Electrically operating selecting systems



Dec. 13, 1955 ELECTRICALLY OPERATING SELECTING SYSTEMS Filed May 7, 195] 10 Sheets-Sheet l Pl E J R? CL P2] 2?! LML/ LML?

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ELECTRICALLY OPERATING SELECTING SYSTEMS Filed May '7, 195] 10 Sheets-Sheet 2 Fla. 4.

Dec. 13, 1955 T. H. FLOWERS ETAL 2,727,094

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Dec. 13, 1955 T. H. FLOWERS ET AL ELECTRICALLY OPERATING SELECTING SYSTEMS 1O Sheets-Sheet 5 Filed May 7 1951 5 8? r w 9 J. n m m; L 38+ 4 39 wk o 38+ 0 m 2? 87 In 88+ r 5- m W I Q :m smw 23 m 8 2g mi 5 m2 @w% m 10 Sheets-Sheet 6 T. H. FLOWERS ETAL ELECTRICALLY OPERATING SELECTING SYSTEMS Dec. 13, 1955 Filed May '7, 1951 Dec. 13, 1955 T. H. FLOWERS EI'AL 2,727,094

ELECTRICALLY OPERATING SELECTING SYSTEMS Filed May 7, 1951 10 Sheets-Sheet 8 Fll.

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Dec. 13, 1955 T. H. FLOWERS ETAL ELECTRICALLY OPERATING SELECTING SYSTEMS l0 Sheets-Sheet 9 Filed May 7, 195] 10 Sheets-Sheet 10 Dec. 13, 1955 T. H. FLOWERS ETAL EILECTRICALLY OPERATING SELECTING SYSTEMS Filed May 7. 1951 United States Patent 2,727,094 ELECTRICALLY OPERATING SELECTING SYSTEMS Thomas Harold Flowers, Mill Hill, Middlescx, Samuel Denis Harper, Prestbury, Cheltenham, Gloucestershire, and Lionel Roy Frank Harris, London, England, assignors to Her Majestys Postmaster General, London, England Application May 7, 1951, Serial No. 224,374 Claims priority, application Great Britain May 17, 1956 25 Claims. (Cl. 17918) This invention relates to electrically operating selecting systems and particularly, although not exclusively, to automatic telephone exchange selecting systems in which in the operation of the system, a selection has to be made automatically of a circuit, switch or the like from among a plurality of circuits, switches or the like all equally suited to some desired operation or function in the system. To facilitate an appreciation of the broader aspects of the invention and an understanding of exemplary embodiments thereof, the present specification is subdivided into sections under the following headings, here tabulated to constitute an index to the specification:

INDEX I. II. III. IV.

Following this order of presentation:

I. PROLOGUE AND DEFINITIONS The invention will be described with-reference to its application to automatic telephone exchanges. In the operation of said exchanges, a calling subscribers line, junction or other line external to the exchange is connected to another subscribers line, junction or other line over, or through the agency (according to the type of exchange) of line-finders, selectors, register-translators and other units of apparatus interconnected by trunks within the exchange, and provision has to be made for many such connections to be set up and maintained simultaneously. The connection of a line or trunk over a switch to another line or trunk involves the indication to the switch of the lines or trunks to be connected and the indication varies according to the characteristics of the switches in use. Crossbar switches, for example, need current in one horizontal and one vertical magnet to indicate a connection to be made over a switch. The problem of selection has several aspects. In one aspect, lines and trunks, collectively referred to as circuits, are provided in groups, and in the course of the establishment of any one connection, selection of a free circuit in a designated group of circuits has to be made at each switching stage in the connection and the selected circuit indicated to the switch which makes the connection.

The group may be designated by the number dialled or other known means but the means of designation does not form part of the present invention. The free circuits in the designated group may be marked as free for selection by well-known means. The lines of a subscriber having more than one line form a P. B. X group. The line of a subscriber having only one line may be regarded for the purpose of description as a group of one line. Groups of one circuit are not excluded from the present invention. A number of groups of circuits as so far defined may be assembled into a larger group for connection to a unit switch or may be re-grouped into a number of further groups each connected to a unit switch. In any arrangement however, the circuits from among which a selection has to be made have to be marked and provided that a group of circuits is understood to be the circuits connected to a unit switch, this aspect of selection may be concisely stated as the selection and indication of one out of a number of marked circuits in a group of circuits.

In a further aspect of selection, in a first group of circuits having access through switches to a second group of circuits for the purpose of establishing connections through the exchange, the circuits in the first group call for connection through the switches to circuits in the second group and when a circuit calls for a connection it is necessary both to select and indicate a free switch and to select and indicate to the said selected switch the selected calling circuit. Two or more circuits in the first group may call substantially simultaneously and it may then be necessary to select the calling circuits one at a time for connection through the switches. The calling circuits indicate or mark themselves by a calling signal. The selection of free switches is accomplished by an apparatus to which the switches are connected over circuits over which each switch is marked when free so that the selection comprises the selection of a free circuit.

Selection and connection by switches in automatic exchanges in its most general form comprises a group of circuits, a number of which are marked as suitable for connection, the selection of one of the marked circuits and the indication of the selected circuit.

Time-spaced trains of pulses are used in selecting means operating in accordance with the present invention and it is convenient to define what is meant by time-spaced trains of pulses. Thus, a pulse is a brief change of current in a circuit, and a train of pulses is a train of rhythmically occurring pulses. A plurality of trains of pulses are said to be time-spaced when the pulses of the trains of pulses occur at diilerent times, that is, no pulse of any one train of pulses overlaps in time a pulse of another train of pulses. Time-spaced trains of pulses are commonly timed so that the pulses of each train of pulses occur in cyclic order with equal time spacing between the successively occurring pulses. Means for generating time-spaced pulses are described in the specifications of copending U. S. Patent applications Ser. No. 55,619, filed October 25, 1948 and Ser. No. 191,584, filed October 23, 1950. v

in this specification the term register is used to denote a bi-stable apparatus which has two stable states of equilibrium, one of which is the normal or unoperated state and the other the operated state. The term includes Eccles-Jordan trigger circuits, gas-discharge tubes and flip-lop circuits as defined in definition 65.06.086 of American Standards Association Communications 42. A register is operated on the occurrence of a set of circuit conditions which are particular to that register and the register remains operated to register that its particular set of conditions has occurred.

An automatic telephone exchange is a switch built up of smaller switches, each of which has two groups of circuits connected to it and when suitably controlled will connect as many as required of the circuits in one group each to a circuit in the other group. Each group of circuits may be subscribers lines, junctions and so forth or trunks between switches. It is common practice for each of the circuits in one of the groups to be terminated on apparatus which when suitably controlled connects that circuit toa selected one of the circuits inthe other group.

In this specification the term connector is used to desig-- nate said apparatus.

According to the presentinvention, apparatus for selecting one of a number of circuits marked out-of a group of circuits comprises means for generating a time spaced train of marking pulses for each of the circuits in'the said group of circuits, at least one set of registers, the registers of each set being arranged in'atleast one group, the number of combinations of one register operated in each group being at least equal tothe number of circuits in the said group of circuits, means for indicating a different circuit in said group of circuits by each combination of one register operated in each group of registers of each said set of registers, means for marking a free set of registers, means for applying the trains of pulses of the said circuits marked out of said group ofcircuits to means responsive to one of the marking pulses applied to it to operate in the said marked set of registers, the combination of one register in each said group of reg isters indicating the circuit the train of marking pulses for which includes the said one of the marking pulses.

Apparatus for selecting one of a number of circuits marked out of a group of circuits further comprises, according to the invention, means. for generating a timespaced train of pulses for each of the circuits in the said group of circuits, a master set and at least one slave set of registers, the registers of each master and slave set being arranged in at least one group, the number of combinations of one register operated in each group being at least equal to the number of circuits in the said group of circuits, means for indicating a difierent circuit in said group of circuits by each combination of one register operated in each group of registers of each said slave set of registers, means for marking a free slave set of registers, means for applying to the said. master set of register the train of pulses of the said, circuits marked out of said group of circuits, means for operating a combination of one registering means in each said group of register in the said master set of registers by a pulse of said train of pulses applied to said master set of registers, and means for operating, registers in the. said marked slave. set of registers means corresponding to the said operated register means in said master set of registers.

The invention further comprises means to prevent the. operation of further registers in a set of registers. after the operation of one register in a group of registers has been initiated or completed.

II. DRAWINGS In order that the invention may be more fully described, systems arranged to operate in accordance therewith will now be described in greater detail with reference to Figs. 1 to 8 of the accompanying drawings which illustrate such systems diagrammatically, and Figures 10 to which when assembled as shown in Fig. 16 show the circuits of a particular application'of the invention to a telephone switching system, Fig. 9 showing pulses suitable for operating the circuits of Figs. 10 to 15.

HI. GENERAL DESCRIPTION Referring to Fig. l, a group of circuits CT, of which three CTl, CT2 and CT3 are shown, are connected to line-finders LFI, LF2 and LF3 respectively in a telephone switching system. The line-finders have connected to them, but not shown in the figure, a group of incoming circuits any one or mo e of which may call for connection to a free circuit CTl, CTZ or GT3 over the'asso- 4 ciated line-finder. When any incoming circuit calls for a connection to one of the circuits CT, a signal-is given in known manner over a common lead ST, and it is then required to select a free circuit and line-finder to connect to the calling incoming circuit. Marking leads ML of which three, MLl, ML2, ML3, are shown connect the line-finders to a generator of time-spaced trains of marking pulses MX and over each lead is marked i. e. signalled whether the circuit CTl, CT2 or CT3. and the line-finder to which it is connected is free or already engaged on a connection. The pulse. generator MX generates a time-spaced train of marking pulses for each marking lead which marks the free condition of its linefinder, andapplies all the pulses generated over the common lead CL to the registers R1, R2 and R3. Registers R1, R2 and R3 comprise a set of registers in one group. Each register is connected to a line-finderover one of a number of leads N of which N1, N2 and N3 are shown and when operated indicates by a signal overthe'lead that the line-finder andcircuit CT towhich it is-connected is selected to make connection to the callingline; Eachregister' has supplied to it a train of registering pulses p, the references pl, 122' and p3 being'applied' to the connecting leads to the registers, the pulses of the train of pulses coinciding with the pulses of the train ofmarking pulses generated by the generator 'MX when" the marking lead ML of the line-finderto which the regis= ter is also connected marks the free condition of the linefinder. incoming circuit, each registerassociated with a tree linefinder will receive pulses overlead CL coinciding with the pulses of its pulse source 7. When an incoming circuit calls, a signal is given over lead ST as already described, and is communicated to each register R, thus marking the set of registers. A register will operate and remain operated if it receives simultaneously a signal over lead ST, a pulse from the generator of its pulse p, and a pulse over lead CL. Because of the time'spacing of the marking pulses generated by pulse generator MX, no two registers can operate simultaneously. The registers are arranged to operate bymeans not shown, so that when one is operated no second register can operate. Hence in response to-thesignal, over lead ST, a free circuit and line-finder, if there is a free circuit and linefinder, is selected and marked to connect, the circuit through the line-finder to a calling line. The time-spaced pulse generator MX may be, for example, a multiplex modulator of the type disclosed inthe specification c0.- pending of patent application SerialNo; 191,584. The registers R may be. tor example, three electrode. gasdischarge tubes. with the signal overlead ST, and pulses p and CL communicated to the strikers so that each tube will fire when a signal overlead ST, and a pulse of its pulse train. p. and CL simultaneously exist. A tube when fired can prevent, for example, by: a resistor common to all the cathodes, any other tube from firing. The signal from .a reg-ister'over its: lead N to'indicate the selected :l-ine finder and circuit may be a D. C. signal orany other convenient signal.

When a free. linefinder has been selected and has responded to the.indicaung.signaloverits. lead N that it is selected, it is necessary 'to release the operated registers so. that a further selection may bemadc; .The means of' releasing the operated registers .arernot shown in the figure but may comprise an apparatus'associated with each line finder, the said apparatus emitting a release signal to the set of registers when the liner-finder is marked over its N lead but signalled as: busy over its marking Iead ML. A

line-finder marked over-itsN lead will normally-find a calling line and thus be signalled busy andbrin-gabout the release of the registers. Exceptionally, in the even-t of a fault for example, the marked line-finder will not be marked busy. It may therefore be desirable to release the registers a fixed time after operatiorg'the 'time'being longer than necessarytfor normal operation: Inthe event Normally therefore in the absence of a calling of timed release, a further selection will take place if the signal over lead ST is still in existence, and there is then a high probability that a different line-finder will be selected.

Fig. 1 shows a group of three circuits CT for the purpose of illustration. Clearly the group may comprise any number of circuits for which time-spaced trains of pulses may be generated. Fig. 1 also shows only a single group of registers equal in number to the number of circuits CT. When the number of circuits is large, it may be undesirable to provide an equal number of registers. An economy in registers can be made by using more than one group of registers, the combinations of one register operated in each group being at least equal to the number of circuits instead or the number of registers being equal to the number of circuits as illustrated in Fig. 2.

In Fig. 2 a group of line-finders, circuits and marker leads are represented by one line-finder LF, circuit CT and marking lead ML connected to the time-spaced pulse generator MX. Any one of the line-finders and circuits which is free will signal the free condition over its lead ML and cause the pulse generator MX to generate a time-spaced train of marking pulses, all the pulses being emitted over a common lead CL. in this example two groups of registers are provided and each register is connected to the common lead CL and to a registering pulse supply. One group comprises the registers R11, R12 Rm connected respectively to a registering pulse supply p11, p12 pin. The second group comprises the registers R21, R22 Ram connected respectively to a registering pulse supply p21, p22 P2M.

The number of groups of registers and the numbers of registers in each group are chosen so that the number of combinations of one register in each group is at least equal to the number of line-finders and circuits in the group of line-finders and circuits. The trains of registering pulses applied to the registers and their time-spacing are arranged so that for each train of marking pulses which the generator MX may generate, the pulses of the train of marking pulses coincide in time with a pulse from one pulse supply each of the groups of pulses. This may conveniently be arranged by using for the pulse generator laiX a multiplex pulse modulator of the types disclosed in the specifications of co-pending U. S. patent applications Ser. No. 191,584, filed October 23, 1950 and 212,736, filed February 26, i951, Patent No. 2,632,880, issued March 24, 1953, the modulator being fed from the same 2 pulse sources as are connected to the registers. An arrangement of this kind is described later in this specification with reference to Figs. 9 to 15. Each register is arranged to be operated and thereafter to remain operated by the coincidence of a signal over a lead ST connected to all the registers, a pulse over lead CL and a pulse from its ,0 pulse source. Hence in response to a signal over lead ST, if there is a free line-finder and circuit CT producing a marking pulse over the common lead CL, one register in each group of registers is operated. It is arranged that when a register is operated no other register in the same group can operate. Each register is connected over leads, for exampe, N111 and N211, to a number of line-finders, each of which is connected to one register in each group of registers and in such order that each line-finder is connected to a difierent combination of one register in each group of registers. A line-tinder is marked as selected to connect its circuit through to a calling line when all the registers to which it is connected are operated.

Referring to Fig. 3, L1, L2 LN represent a group or telephone lines; each telephone line may consist of a number of Wires but is shown diagrammatically as a' single lead. The lines are connected in multiple to a number of connector switches, of which two SW1 and SW2 are shown in the figure. Each switch has connected to it a circuit CT and forms part of a line-finder, the

sets of registers RSW, of. which RSWl and RSW2 are represented forming another part. The lines L signal by a calling signal when connection through a line-finder is required. The connector switch of a selected line-finder is required to make a connection between a calling line, or if more than one calling line exists, of one of the calling lines, to the circuit CT connected to the line-finder. Each switch SW and its associated set of registers RSW are connected over a group or groups of leads N, of which N1 and N2 are represented, each lead connecting a register with a controlling element in the switch. The number of groups of registers in a set, and the numbers of registers in the groups is chosen so that the number of combinations of one register operated in each group of registers is at least equal to the number of lines in the group of lines L1, L2 LN. Each connector switch SW is constructed so that each of said combinations signalled over the leads N causes a particular line to be connected to the circuit CT. Switches of the type described are known and referred to more particularly later in this specification.

Each line L is connected to an apparatus LC of which examples LC LCZ and LCN are represented, which emits over one of a number of marking leads LML of which LMLl, LMLZ and LMLN are shown, a signal when the line is calling for a connection through a linefiner, but is not so connected. The marking leads LML are connected to a generator or" time-spaced trains of pulses MX which generates and applies over the common lead CL 3. train of pulses for each calling circuit as described with reference to Figs. 1 and 2 for the generation of time-spaced trains of pulses for the free linelinders. The common lead CL is connected to all the sets of registers RSW, which are connected to a set of registering pulses p also as previously described. Each set of registers signals over a marking lead ML to a line-finder selector LEA the condition free or engaged of the set of registers. The linedind-er selector LEA is connected to each set of registers over a lead STN (STiJl, STNZ, etc.) and in response to a signal over the lead ST marks the STN lead of a free set of registers.

The apparatus LEA may comprise for example apparatus according to Fig. l or 2. As shown in Fig. 3, the lead ST is connected via a pulse lengthener PL to the common lead CL. A pulse over the lead CL is lengthened by PL into a continuous signal which when applied to lead ST causes LEA to select and mark over its lead STN a free line-finder as previously described.

in operation, when one of the lines L calls, the marking lead LML of the calling line is marked and a timespaced train of pulses appears over lead CL. The train of pulses over lead CL is lengthened by pulse lengthener EL to a continuous signal applied over lead ST to the line-finder selector LPA which selects and marks over its lead STN a free set of egisters RSW. Registers in the set of registers RSW marked over its lead STN, operate to one of the pulses over lead CL, and indicate to the connector switch SW by the combination of leads N which are marked which calling line to connect to the circuit CT. if more than one line L calls simultaneously, the marked set of registers RSW will select one of the lines, another set of registers will then be marked over its lead STN and select another calling line and so on. The registers in any one set are released when the line and circuit connected through the switch SW are released.

Fig. 4 shows an arrangement similar in many respects to that of Fig. 3 and in which the sim lar parts have the same designations as in Fig. 3. It is suitable however for the operation that a circuit CT has to be connected to a free line from amongst a'number of lines out of the group of lines L. Lines which are suitable for connection to the circuit CT are marked by a marker M, over marking leads LML of which there is one for each circuit L. Suitable lines are free lines in a designated group of lines and the marker M may be the type of marker well known in crossbar switching systems and adapted as will be readily apparent to those skilled in the art to mark the free lines in a designated group of lines. In this arrangement the set of registers is marked over its lead-STN from a signal transmitted over the. circuit CT, shown schematically in Fig. 4 by a connection between each circuit CT and its marking lead STN. In operation, the marker M marks the leads LML of the linesL suitable for connection, the train of pulses corresponding to the marked leads are transmitted over the common lead CL to all the sets of registers, the circuit to which a line L is to be connected marks its set of registers over its STN lead and a register in each group in the marked set of registers operates to one of the pulses over the lead CL, thus selecting one of the marked lines L for connection. The operated registers mark their leads N and thus control the switch SW to connect the circuit CT to the selected line L andare released when the line and circuit are released.

The telephone connector switches. which are shown schematically and marked SW in Figs. 3 and 4, preferably have the capacity to switch a large number, for example 100, lines L to a circuit CT. The number of registers in a set of registers may therefore be of the order of twenty, for example, two groups each of ten registers.

Each register has to have associated with it a gate. circuit which responds to the simultaneous existence of a marking signal over the lead STN, a pulse over lead CL and a pulse over a p pulse lead, and because the gate circuits and registers are operated by pulses they have to be relatively quick acting and may therefore be expensive to provide compared with slower acting devices, although cheaper and slower acting devices might be adequate to control the switches.

According to a feature of the invention not shown in the diagram, the registers may be relatively slow-operating devices, for example, gas-discharge tubes, provided that impulse lengtheners are interposed between the gate circuits and the registers and that means are also provided to render inoperative any further pulse over lead CL which follows an operative pulse within a time interval at least equal to the operating time of the, registers. These means may comprise, for example, a blocking oscillator or pulse trigger circuit interposed between the lead CL and the sets of registers, and the pulse lengtheners may comprise for example capacitors charged through rectifiers.

An economy may be achieved. by providing one master set of registers connected in multiple to a plurality of slave sets of registers, operated registers in the master set of registers operating corresponding registers in a marked set of slave, registers. This. isv explained in more detail with reference to Fig. 5.

In Fig. 5, MX is a.v generator of time spaced trains of pulses corresponding to the similar generators MX in the previous figures. pulses generated by the generator MX are communicated to a master set of registers RM. To the master set of registers is connected the p pulse supplies. corresponding to the p pulse supplies of Figs. 3 and 4. Each register in the master set operates when a registering pulse from the pulse source connected to it coincides with a marking pulse over the common lead CL, one register in each group of registers being operated and the; operation of further registers prevented as described previously. is not essential for the. master set of .registers to be controlled by a marking lead ST or STN of the previous figures. A register in the master set when operated, marks one of the leadsof a group of leads RML connected to all the slave sets of registers RSW (RSWl, RSW2 etc) CL is a common lead over which the scribed. Because the gate circuits and-registers 'of the slaves are. not operated by timedpulses they may be less complicated and expensivethan those of the master. It is necessary to release the operated registers in the master set of registers. sothat a further selection may be made when required.

If theapparatus. of Fig. 5 is applied as a modification to Fig- 3, the master set of registers will mark over the leads. RML a circuit which is marking the pulse generator 'MX.v When a slave set of registers responds to the marking, a line-finder should connect to .the marked calling circuit and remove thev marking signal from MX. By a suitable combination of the markings over leads RML and the marking leads to the generator MX, the master setof registersv may bemade to release when a marked calling circuit has been picked up by a line-finder, or bya timed release, as described in connection with Fig. 1. If the apparatus of Fig. 5 is applied as a modification to Fig. 4, the master set of registers may be released when the marking pulses over lead CL cease.

In Figs. 3 and 4,. switches SW are describedas being controlled. to .connect a circuit CT to a line L when one lead in each group of leads in the set of leads N is marked. If the switch SW is a mechanical switch of known type, for example the so called motor switch, having a, set of wipers rotating over a bank of contacts, markings of the type described are a well-known means of positioning the wipersv on. a particular contact. In Fig. 6 which represents a switch having eight contacts in the bank, as an example, two levels SL (SL1, SL2) of contacts are each traversed bya wiper W (W1, W2). The contacts of level SL1 are commoned in two groupsof four contacts each and connected thro gh a. relay contact C (C1, C2) and a resistor to the exchange battery. The contacts of level SL2 are commoned in four groups of two contacts, each group being connected via a relaycontact D (D1, D2, D3, D4) to earth. The two wipers are connected together via a relay HS. The break contact of the relay HS is connected on one side to a start lead STN and to the other side to a latch magnet in series with a battery.

-In operation, one contact in each group of contacts C and D is operated, a start signal is applied to the lead STN, the latch magnet LM operates and in known manner causes the wipers to rotate. When current flows through relay HS therotation of the wipers is arrested Serial No. 56,619.. discloses a connector switch which comprises time-spaced. pulse generators to provide connections between a group of circuits onone side of the switch and circuits. on the other side of the switch. The present invention is readily applied to the, control of. switches of this type as will now be described with reference to Figs.

7 and 8.,

Referring first to Fig. 7, a line-finder connector switch described in the specification referred to is schematically represented by the part of they figure above the dashed line. L1, L2 LN are'lines connected to a. generator and modulator of time-spaced trains of. pulses. MLX. Each line receivescalling and clearing signals and speech signals from apparatus not shown. The modulator MLX provides over a common signal circuit CH 2. time-spaced channel train of pulses for each line which is receiving a calling signal, and the speech over the line modulates the channelpulses. Switches SW1, SW2 are connected to the common. signal circuit and each. comprises a gate circuit. G (G1,, G2.) which, when operated by a pulsewhich will be termed. a selecting pulse over'a lead SP (SP1, SP2) connects the common circuit CH to a selector circuit CT(CT.1, .CT2.)., which comprises means not; shownfor demodulatinggand amplifying a modulated pulse. A selector circuit is thus eonnectedto any'line L when there is connected to the lead SP a train of selecting pulses coinciding with the train of channel pulses produced by the modulator MLX in response to the calling signal over the line L. A pulse suppression gate circuit PS8 is connected to the common signal circuit CH and to the SP leads via de-coupling means, not shown, of all the switches SW. The action of the gate circuit PSG is that the circuit CH is connected to a circuit CPH if no pulse is present on any SP lead but is disconnected for the duration of any pulse transmitted over any SP lead. Hence the train of channel pulses of any line L which is calling but is not connected to a circuit CT appears over iead CPH. These pulses may be applied directly to the sets of registers RSW, to which are also applied registering pulses p in groups and having the same time relationship with the pulses generated by the modulator MLX as described for Fig. 2 for the pulses p in that figure had in relation to the pulses generated by the generator MX. A mark on a lead STN to a set of registers will cause the registers in that set to respond as previously described. The operated registers in this case gate trains of pulses, the p pulses for example, which are combined into a single train of pulses are applied over lead S? as required by the selector. Alternatively, one master set of registers may operate to the pulses over the lead CPH and control the operation of registers in a marked slave set of registers which will produce a required selecting train of pulses over its lead SP. Release of the master set of registers is advantageously caused by comparing a train of pulses generated by the operated registers in the master set of registers with the trains of pulses over the lead CPI-l. When the master set of registers first responds to the marking pulses, it generates a train of pulses synchronous with one of the trains of marking pulses, over the lead CPH. When a slave set of registers has responded to the master set markings, the marking train of pulses coinciding with the train of pulses generated by the master set of registers disappears from the circuit CPH and this may be made to release the master set of registers.

Fig. 8 shows a selector connector switch in which calling circuits CT (CTl, CT2) are connected through gate circuits G (G1, G2) to a common signal circuit CH connected to a multiplex modulator MLX which distributes time-spaced trains of pulses on the circuit CH to the lines L1, L2 LN. The gate circuits G are operated by selecting pulses over leads SP which are connected as in Fig. 7 to a pulse suppression gate circuit PSG also as in Fig. 7. The gate circuit PS6 is connected, however, not to the channel CH, but to the output of a marker multiplex MMY. The marker multiplex has connected to it a marker lead ML for each line L connected to the multiplex MLX and in response to a marker signal on any one lead ML provides over the output circuit a train of pulses coincident with the train of channel pulses through multiplex MLX of the line L corresponding to the selected lead ML. The group of lines from which a free one is to be selected for connection to a calling circuit CT may be indicated over the marker leads ML by means described in the specifications of co-pending patent applications Serial Nos. 56,619 and 177,411. The train of pulses of any line already connected to a circuit CT is suppressed at the pulse suppression gate PS6. The lines L which may be connected are thus indicated by the trains of pulses which are communicated over the circuit CPH to the sets of registers RSW. The set of registers required to respond to one of the pulses is marked over its lead STN by the calling signal over the circuit CT shown schematically as connected'to the lead STN. The generation of the selecting pulses over the lead SP and the alternative of master and slave sets of registers is similar to that described in connection with Fig. 7.

IV. Exemplary embodiment Figures 9, l0, 11, 12, l3, l4 and 15 show by way 10 of example and in greater detail, one form that may be taken by the apparatus already described in outline.

Fig. 9 shows the time distribution of the various pulse trains used in this embodiment of the invention. The pulse train designated KS1 is of one microsecond pulse repetition time, the pulses of which are positive going, substantially rectangular and of duration 0.2 microsecond. The pulse train designated KS2 is of one microsecond pulse repetition time, the pulses of which are negative going, substantially rectangular and of duration 0.2 microsecond. The KS2 pulses occur approximately 0.5 microsecond after the KS1 pulses. The pulse train designated KS3 is of one microsecond pulse repetition time, the pulses of which are positive going, substantially rectangular and of duration 0.2 microsecond. The KS3 pulses occur approximately 0.6 microsecond after the list pulses. These pulse trains KS1, KS2 and KS3 may be produced using means well known to those skilled in the art, comprising for example a one megacycle per second oscillator, limiting amplifiers, differentiating circuits and the like.

Four other groups of pulse trains are used. in one group there are 11 pulse trains, three of which, designated 1 P 7 and 1 are shown in the diagram. These pulse trains each have a pulse repetition time of 11 microseconds and the pulses of all the pulse trains in the group are equally spaced in time and are positive going. Thus pulses of pulse train P f occur one microsecond before the pulses of pulse train P the pulses of 1 occur one microsecond before the pulses of P' etc. In the other group there are 9 pulse trains, three of which, designated P%, 1 and P%, are shown in Fig. 9. These pulse trains each have a pulse repetition time of 9 microseconds and the pulses of all the pulse trains in the group are equally spaced in time and are positive going. Thus pulses of pulse train 1 occur one microsecond before the pulses of pulse train P%; the pulses of 1 occur one microsecond before the pulses of 1 etc. in both these groups every pulse is substan tially rectangular and of duration 0.8 microsecond and is centered on a KS1 pulse.

Fig. 9 also shows negative going versions of the above two groups of pulses which are used in the applications to be described. These are designated by the sulfur n.

Thus the negative going version of P is referred to as P 5 1: etc.

A group of pulses occurring every 9 microseconds is herein referred to as a 9-phase group of pulses and any one of the pulse trains of the group is referred to as a 9-phase pulse train. Similarly the pulses occurring every ll microseconds are referred to as being ll-phase pulses or ll-phase pulse trains of an ll-phase group.

it is assumed that all the aforementioned pulses are available in a low impedance of, for example, '75 or ohms and are of suitable amplitude, for example, 29 volts. The 9-phase and the ll-phase pulses may be derived using, for example, pulse frequency dividing means described in co-pending patent application Serial No. 212,736 and pulse amplifiers etc., well known to those skilled in the art.

Figures 10 and 11 show the apparatus used to transmit speech and hold signals through a time-division multiplex switch and the means for marking the circuits on the multiplex side of the switch; Figure 10 shows the apparatus associated with circuits on the multiplex side of the switch and Figure 11 shows a connector switch associated with circuits on the connector side of the switch.

(A) l /lultiplex side circuit arrangements (Fig. 10)

In Figure 10 apparatus to the right of the vertical dashed line, and to the left of commoning points M1, M2 and M3 is associated with each circuit connected to the multiplex side of the switch, apparatus that lies between points M and N is common to a number of such circuits and apparatus that lies to the right of points N1, N2 and N3 is commonto the switch. Each of the circuits connected to the multiplex side of the switch usesan individual train of channel pulses. for the transmission of speech and hold signals thronghthe switch and the pulses of a channel pulse-train are produced on the coincidence.

of the pulses of two pulse-trains, selected one from the 9-phase group of pulses and the other from the ll-phase group of pulses. In Figure 10 the circuit illustrated uses the two pulses P /Q and P /u to generate its channel pulses. There are 99 combinations of x from 1 to 9 with y from 1 to ll, so that 99 circuitsv may be. connected to the multiplex.

Speech signals into the switch are applied over pair circuit 1 to transformer T1 and the potential difference between the plates of capacitor C3 follow the voltage of the applied speech signals. A hold signal is applied either as a 10 kc./s. signal on pair circuit 2 or as a uni-.

directional signal on lead accordingto the usage of the switch inthe exchange systems; As shown in Fig. 10, a D. C. hold signal is applied; overload 5 from a sub-. scribers line circuit including resistor R18; When a hold signal is applied over lead 5, the A. C. hold circuit transformer T2, rectifiers W9 and W10, resistor R5 and capacity C4 are omitted; In other usage of the switch, for example when the circuits 1, 2, 3 and. 4 are connected over a trunk to circuits of another switch, an A. C. hold signal is applied over circuit 2 and lead 5. and rectifier W11 are omitted. When a hold. signal of either kind is applied the junction between rectifiers W1, W2 and W3 may rise to a potentialof some: 8 volts. above earth potential.

In the absence of a hold signal and a pulse on either of the leads connected to capacitors Cl'and C2 each of the two equal currents flowing through resistors R1 and R2 is arranged to exceed the current flowing through resistor R3; a current, equal to the difference between the sum of the currents through resistors R1 and R2 and the current through resistor R3 flows-through rectifier W3 in its conducting direction. The potential of the junction between rectifiers W1, W2 and W3 is then slightly below earth potential. Each positive going pulse from pulse source P /g (or pulse source P /n) biases rectifier W1 (or rectifier W2) to its high resistance state so cutting otf the current-through the rectifier W1 (or rectifier'WZ); the current through resistor-R2 (or resistor R1) exceeds the current through resistor R3 so that a current still flows through rectifier W3 in its conducting direction and the potential of the junction between rectifiers W1, W2 and W3 is still slightly below earth potential. When one of the pulses P /Q coincides with one of the pulses P 11 both rectifiers W1 and W2 are biased to their high resistance states and current ceases to flow through the two rectifiers; in the absence of a hold signal the current from the +50 V. supply flows via resistor R3 and rectifiers W10 or W11 to charge the capacitance of capacitor C4 or through the load impedance that connects lead 5 to earth and the potential of thejunction between the rectifiers W1, W2 and W3 'does not rise appreciably above earth potential.

When a hold signal is applied to pair circuit 2 or to lead 5, capacitor C4 is charged, so that the potential of its upper plate is at least 8 volts above. earth potential or the potential left hand terminal of rectifier W11 is at least 8 volts above earth potentiahthen, when. the pulses of pulses P /s and pulses P /n coincide the current from the +50 V. supply flows through resistor R3v and rectifiers W4 and W6 to charge the self-capacitances of the leads between the junction of rectifiers W1, W2 and W3 and the grid of valve V1.

In the absence of speech signals on pair circuit 1 the potential of the upper plate capacitor; C3 .is -5 volts above earth potential and when the; potential of the self-capaci-. tance of the lead-between rectifiers W1, W2 and W3 and the grid of valve V1 rises to '5 volts, above earth potential, rectifier'WS. becomesconducting and the-current; through. resistor'mfiowssmainly'into capacitor;C3; thezcapacitanoe' of capacitor is somuch greater than the self-capacitance of the lead between rectifiers W1, W2 and W3 and the grid of valve V1 that substantially all the current from resistor R3 flows into capacitor C3. During the coincidence of the pulses the potential of the grid of valve V1 thus rises to 5 volts above earth potential and after the disappearance of the two coincident pulses the potential of the grid of valve V1 decays towards earth potential mainly through resistor R7. Shortly after the disappearance of the .coincident pulses a negative going pulse appears on pulse lead KS2 and during the presence of those pulses the self capacitance associated with the lead betweenrectifiers. W1, W2 and W3 and the grid of V1 .is discharged, rectifier W7 clamping the final discharged potential of the self capacitance to a value silghtly below earth potential. The lead, after one of the pulses KS2, is thus at a potential whose value does not depend on the amplitudeof a pulse that may have preceded the pulse. Thus inthe presence of a hold signal on either pair circuit. 1 or lead 5 a pulse of 5 volts amplitude appears atthe grid of valve V1 when the pulses of pulses P /s and PW 11 coincide; this they do at intervalsof 99 microseconds and the individual pulses form the ,unmodulated pulses of the channel.

When speech signals. are present on pair circuit 1 the potential above .earth potential of the upper plate capacitor C3 will be 5 volts plus the algebraic instantaneous voltage of the speech signal that appears on the secondary winding of transformer T1; it is so arranged by means of limiters (which are not shown in the figure) that the instantaneous peak voltages. of the speech signals that appear across the resistor R4 do not exceed 2 /2 volts, so that the potential of the upper plate of capacitor C3 may vary between 2% and 7% volts above earth potental. In the presence. of speech signals on pair circuit 1 the pulse that appearsat the grid of valve V1 will be limited not to'5 voltsv but to the voltage of the upper plate of capacitor C3 at theinstant. of the coincidence between one of the pulses P /a and one of the pulses P /n and the channel pulses of the circuit will be amplitude modulated by the speech signals that appear on pair circuit 1.

The apparatus between pairs 1 and 2 and lead 5 and point M1 form what is known as a pulse modulator. Pulse modulators, utilising other combinations of P /s and P /n pulse supplies may be connected in parallel at. point M1 and similar groups of pulse modulators may be connected together at point M1, rectifiers W4 and W6 acting as decoupling components.

Valve V1 is a cathode follower amplifier whose output circuit is connected to the grid of valve V2 (another cathode follower amplifier) through a pulse modulator comprising-resistors R9, R10 and R11, capacitor C7- and rectifiers W12, W13 and W14. In the absence of pulses fromlead KS1 or capacitor C6, each of the equal currents through resistors R9 and R10 to the --150 V. source exceeds the current-from the +200 V. source via resistor R11 and the difference between the sum of the currents through resistors R9 and R10 and the current through resistor R11 flows through rectifier W14 in its conducting direction; the potential of the grid of valve V2 is then slightly below earth potential. A positive going pulse of pulses from KS1 or from cathode follower amplifier V1 will cut oh the current through resistors R10 or R9 but current will still fi'ow through rectifier W14 in its conducting direction. When one of the pulses KS1 coincides with a pulse from cathode follower amplifier V1 the current from the +200 V. sourcethrough resistor R11 charges the self-capacitance of the lead between resistor R8 and the grid of valve-V2. The pulse amplitude of pulses KS1 greatly exceeds the maximum modulated pulse amplitude of 7 /2 volts that may'appearat the output of cathode follower amplifier V1. The potential of the grid of valve V2 rises during coincidence of a pulse =from-the output of cathode follower amplifier V1 and. enact the pulses KSIuntil the grid potential is equal to the amplitude of the pulse at'theoutput of cathode follower amplifier V1; when that poten tial is reached current from the +200 V. source flows via rectifier W12 to the low output impedance of cathode follower amplifier V1 so that the pulse that appears at the grid of valve V2 has the same amplitude as the pulse from the cathode follower amplifier V1 and the dura tion of the pulses KS1, since the duration of the pulses KS1 is of the order of 0.2 microsecond whereas the duration of the pulses from the cathode follower amplifier V1 is of the order of 0.8 microsecond. The pulses KS1 occur every microsecond and are used as shaping pulses for the channel pulses that appear on lead 8: by this means all channel pulses will have the same duration from whatever combination of pulses Pls and PW 11 they are derived.

The circuit to the left of the vertical dashed line of Figure 10 shows one way in which speech and hold sig nals from a 2-wire subscribers line may be connected to a pulse modulator. The subscribers 2-wire line is connected to a hybrid transformer T6 which has a balancing network BAL. To the centre point of the 2-wire winding of that transformer are connected capacitor C16 and the exchange supply +50 V., R18 and earth. When the subscriber is disengaged no current flows from the +50 V. supply through resistor R18 and the upper terminal of resistor R18 is at earth potential. Lead is connected to the upper end of resistor R18 and whilst the subscriber is disengaged the earth potential on lead 5 prevents channel pulses of the circuit appearing on lead 8. When the subscriber is engaged current from the 50 V. supply flows through the subscribers instrument and his line and hence through resistor R18 and the potential of the upper end of that resistor is arranged to rise at least 8 volts above earth potential. When the potential of lead 5 rises by that voltage, channel pulses of the circuit appear on lead 8. When a subscribers line is connected to a pulse modulator the circuit associated with pair circuit 2 need not be provided.

The receive portion of the circuit is shown above the transmit portion which has just'been described. To lead 7 are connected pulse modulators which will be described more fully later, but which are essentially similar to that already described; the circuit elements associated with valves V3 and V4 and lying between lead 7 and point N2 perform the discharging and pulse shaping duties which are performed by the circuit elements that lie between point N1 and lead 8. Points N2 and M2 are paralleling points similar to point N1 and M1 and rectifiers W15 and W16 act as decoupling elements as did rectifiers W6 and W4. Once again each of the equal currents through resistors R13 and R14 exceeds the current through resistor R15 and the difference between the sum of the currents through resistors R13 and R14 and current through resistor R15 flows in the conducting direction through rectifier W19 so that the potential of the junction between rectifiers W17, W18 and W19 is slightly below earth potential. When coincident pulses appear from pulses P /Q and P /u the currents through rectifiers W17 and W18 are cut oil and the current through resistor R15 charges capacitor C until the potential of its upper plate is equal to the output potential of cathode follower amplifier V4; when that potential is reached the current through resistor R flows through rectifiers W16 and W15 to the low output impedance of the cathode follower amplifier V4 so that the potential of the upper plate of capacitor C10 is made equal to the potential of the pulse that coincides with the simultaneous coincidence of pulses P /s and P /n; diode V5, clamps the potential of the lower plate of capacitor C10 to 4 volts above earth potential. When the pulse at the output of the cathode follower V4 disappears, the potential of the upper and lower plates of capacitor C10 drops by 2. voltage equal to the amplitude of the pulse from the cathode follower amplifier V4 and the potential of the lower plate circuit 2 or lead 5.

thus drops by that voltage from the clamping voltage of 4 volts above earth potential. The potential of the lower plate of capacitor C10 then charges slowly via resistor R16 towards earth potential until the next channel pulse which coincides with the next coincidence between pulses Thus the potential of the lower P /s and P /il occurs. plate of capacitor C10 has a negative going saw tooth waveform whose amplitude is equal to the pulse amplitude of the channel pulses at the output of the cathode follower amplifier V4 that coincide with the coincidences of pulses P /s and P /n. If no channel pulse coincides with the coincidences of pulses P 9 and PW 11 the potential of the lower plate of capacitor C10 remains at earth potential. If the channel pulse is modulated by speech signals then the saw tooth waveform will be similarly modulated in amplitude. Valve V6 amplifies the voltage that is applied to its grid-cathode circuit and in its anode circuit are connected transformers T3 and T4. Transformer T4 is connected to a low-pass filter F1 which cuts off at one half of the 10.1 kc./s. pulse repetition frequency of channel pulses; in pair circuit 3 appear the speech signals that were used to modulate the channel pulses that appeared on lead 7. Transformer T3 is tuned by capacitor C11 to resonate at the pulse repetition frequency (10.1 kc./s.) of the channel pulses; when channel pulses which coincide with coincidences of pulses P /s and P /n appear on lead 7, a 10 kc./s. signal appears across the secondary winding of tuned transformer T3 and that 10 kc./s. signal is rectified by rectifier W20 to bias the grid of valve V7 to its operating state from the non-operating state at which it was held by the connection of the secondary winding of tuned transformer T3 to the l0 V. supply. When the valve V7 is biased to its operating state it oscillates by reason of the backcoupled transformer T5 which is tuned by capacitor C13 to a suitable frequency, here assumed to be 10 kc./s., and a 10 kc./s. signal appears on pair circuit 4. The appearance of a 10 kc./s. signal on pair circuit 4 depends on the appearance of channel pulses on lead 7 which coincide with coincidences of pulses P /s and PW 11 and the appearance of channel pulses on lead 7 will be shown later to depend on the presence on pair circuit 13 (Fig. 11) of a hold signal. To the left of the vertical dashed line (Fig. 10) pair circuit 3 is shown connected via hybrid transformer T6 to a subscribers 2-wire line SL.

The apparatus lying between lead 6 and the dotted lead S (Fig. 10, lower right) is similar in its operation to the apparatus in the figure that lies immediately above it. If lead 6 is at earth potential no pulses appear at the position S but if the potential of lead 6 is raised above earth potential pulses appear at S on the coincidence of pulses P /s and P /u. The pulses that appear at S are known as marker pulses and they coincide with the pulses that appear on lead 8 when a hold signal is applied to either pair Valves V10 and V11 form a transformer coupled amplifier with a transformer output (T8); the first stage valve is a pentode (V10) whose suppressor grid is connected to lead 9. A negative potential on lead 9 prevents valve V10 from passing anode current and therefore inhibits the amplifying function of the amplifier. The use of the amplifier and the apparatus connected to lead 6 will be described later.

B. Connector side circuit arrangements (Fig. 11)

In Figure 11 ar shown the apparatus associated with a circuit on the connector side of the switch and apparatus common to the switch.

Each circuit connected to the connector side of the switch can be connected through the common leads 7 and 8 to any one of the circuits connected to the multiplex side of the switch by applying to lead 11 pulses that coincide with the channel pulses of the multiplex side circuit to which the connector side circuit is to be connected, i. e. a selector pulse train. The pulse modulator comprises the apparatus between pair circuit 12 and 13 andthe common lead 7; speech signals may beconnected to pair circuit 12 and a 10 kc./s. hold signal may beconnected to pair circuit 13. It is so arranged that in the absence. of a hold signal on pair circuit 13 and a pulse on lead 11 the. current through resistor R20 exceedsthat throughresistor R21, the dilference between those two currentsfiowing through rectifier W24 inits conducting direction. The potential of the junction between rectifiers W23 and W24 is then slightly below earth potential. In the presence of a positive going pulse on lead 11 the-current through rectifier W23 is cut oil? andthe current through resistor R21 flows through rectifier W26 to capacitor C20 and again the potential of the junction between-rectifiers W23 and W24 remains only slightlyabove earth potential. If a hold signal is applied to pair circuit 13 the upper plate of capacitor C20 is charged to a potential of at least 8 volts above earth potential and then, when a pulse appears on lead 11, the current through rect fier W23 is cut oil and the current through resistor R21 charges the self capacitance of the lead connecting rectifiers W22 and W21 to lead 7 until thepotential of that lead equals the potential of the upper plate of capacitor C19. The action of this part of the ,circuit is thus similar to that of the pulse modulator connectingpair circuits 1 and 2 to lead 8, and on lead 7 will-appear channel pulses amplitude modulated by speech signals on pair circuit 12 when a hold signal is applied to pair circuit 13. The apparatus in Figure 11 to the right of points m1, m2 and m3 is individual to one of the circuits connected to the connector side of the switch.

At points m1, m2 and m3 the circuit shown is connected in parallel with other similar circuits to form groups of circuits: and the groups of circuits may be connected in parallel at points n1, I12 and n3, .rectifiers W21, W22, W29, W30, W35 and W36 acting as decoupling elements. The channel pulses that appear on lead 7 pass. through the apparatus between lead 7 and point N2 on Figure 10 in which they are operated upon by the KS1 and KS2 pulses as were .the channel pulses that appeared at point Nlin Fig. 10.

Any unmodulated or modulated channel pulses that appear on .lead 8 and coincide with the pulses on lead 11 will charge capacitor C21 in a way similar to that in which capacitor C10- was. charged. The apparatus to the right of capacitor C21 demodulates the channel pulses and produces in pair circuit 15 copies of the speech signals that were applied to the pair circuit 1 whose channel pulses coincide with the pulses on lead 11, and on pair 16 will appear a 10 kc./s. hold signal whose presence. dependson the presence of a hold signal in pair circuit 2 or on lead of the channel whose chanel pulses coincide with the pulses on lead 11.

The pulses on leads 11 of all the circuits on the connectcr. side of the switch are combined through rectifiers arranged in groups and including rectifiers W36 and W35 and applied to the two stage transformer coupled amplifiers V15, V16. The pulses that appear on lead .9 are arranged to be negative going pulses and these pulses areapplied. to the suppressor grid of valve V16 (Fig. so that no pulses can appear on lead 10 if coincident pulses are present on any of the leads 11 of the circuits connected to the selector side of the switch.

C. Slave selectors (Fig. 12)

Each circuit on the connector side of the switch has associated with it a slave selector which is used to generate the pulses that appear on lead 11; i. e., the selecting pulses.

The operation of the slave selector will now be described with reference to 'Fig. 12. As has already been described each connector circuit is associated with a slave selector which may be caused to generate any of the 99 time-spaced pulse trains on a lead, for example lead 11 which is connected to the selector gatesof the associated circuit. Each slave selector includes two groups of registers which in the embodiment of the invention 16 shown in Fig. 12 are cold cathode- ,gas discharge tubes or thyratrons. One group consists of 9 such tbyratrons, three of which V19, V and V21. are shown in Fig. 12. Each thyratron in the group is associated with one of the 9-phase pulse trains. Thus V19 is associated with pulses P V20 with pulses 1 V29 with pulses P% etc.

Similarly the other group consists of 11 such thyratrons, three of which V22, V23 and V24 areshown in Fig. 12. Each thyratron in the group is associated with one of the ll-phase pulse trains. When a slave selector is operated to produce a particular one of the 99 pulse trains, one cold cathode thyratron in each group of cold cathode thyratrons is struck or operated. The operated cold cathode thyratrons are associated with the 9-phase pulse train and the ll-phase pulse train which have coincident pulses coincident with the pulses of the pulse train to be generated. The method of striking the cold cathode thyratrons will be described later in this application.

In the anodecircuitof each .thyratron in either group is situated a gate circuitwhich lefiectively transmits the applied negative going pulses to-a lead common to all the cold cathodethyratrons. in the group, only when the thyratronis operated and conducting. Thus when one cold cathode thyratron in each group is conducting a negative going 9-phase pulse train will appear on the common lead18 of the group of 9 cold cathode thyratrons and a negative going ll-phase pulse train will appear on the common lead 19 of the group of 11 cold cathode thyratrons. two leads. are applied to a coincidence -gate which transmits only the pulses that coincide on leads 18 and 19. The pulses transmitted by the coincidence gate are amplified and transmitted over lead 11 to the connector gates. These are the so-called selecting pulses.

The operation of the coincidence. gate is similar to that of the channel modulator already described in connection with the ,multiplexequiprnent. The common leads 18 and 19 are connected to thelc'oincidence gate via capacitors C and C36, which are connected to the junctions of resistor- R41- and rectifier W44, and resistor R42and W respectively. Substantially equal currents flow through these two resistors and through the low in R43 is substantially less than that flowing in either R41 or R42 .50 that the current normally flowing through the low forward resistance of rectifier W46 is greater than that flowing in either vR41,or R42. The negative going pulses appliedover leads 18 and 19 backptf the rectifiers W44 and during'the pulse so that the current in the rectifier W46fluctuates. These ,currentlfluctuations.

produce only a small voltage change across the rectifier W46 unless there is coincidence between the pulses appliedon leads 18, and 19 when both rectifiers W44 and W45 are backed off and rectifier W46 is also backed ofi. by the current .in resistor R43 for the duration of the pulse. This causes rectifier W47 to conduct and a negative going pulse appears across resistor R44. The rectifier W47 minimises the efiect of small .fiuctuations of voltage across W46. Provided that W46 is conducting W47 will be backed off attenuate the fluctuations transmitted to R44.

The negative going pulse appearing across R44 reduces the current-in triode V17 which is the first valve in a two stage transformer coupled amplifier in which the unwanted fluctuations are finally eliminated by biasing back the grid of the second stage in which full advantage of the duty cycle is taken, The required positive going selecting pulseis thence transmitted via phase reversing pulse transformer T17 over lead 11 to, the selector gates.

The operation of the gates associated with the cold cathode thyratrons'will now" be described. The anode of each coldflcathode thyratron, for example V19 is connected via a resistorlR33 to a rectifier W38 which is con- -17 nected to thecommon lead 18. This commonlead'is connected to the coincidence gate as'already described axid also via R39 to the +85v supply. A negative-going version of a 9 phase-pulse is applied to the junction of R33 and W38 through a capacitor C23. When the associated cold cathode thyratron V19 is not conducting the peaks of the applied pulses cause current to flow through the low forward resistance of rectifier W38 and the resistors R39 and R41 of the coincidence circuit, which chargesthe capacitor C23. Between pulses the capacitor C23 discharges slightly through thehigh reverse resistance of rectifier W38.but the total discharge between pulses is such that the current flowing into C23 during the pulse is substantially less than the current normally flowing in R41. The rectifier W44 is then not backed off during pulses and only small fluctuations are produced on the output of the coincidence gate. When the cold cathode thyratron V19 is conducting the condenser C23 is discharged between pulses not only through the high reverse resistance of rectifier W38 but also through the resistor R33 by the thyratron current. The total discharge of capacitor C23 between pulses is now such that the current flowing into the capacitor C23 during the pulse is greater thanthe current flowing in R41 so that a larger proportion of the applied pulse voltage appears across R39 and the rectifier W44 is backed off, Clearly by striking combinations of one cold cathode thyratron in each group, selecting pulses may be generated .which coincide with the pulses of any of the 99 multiplex pulse trains.

The method of striking the required combination of cold cathode thyratrons will now be described. As already described connections through the switch are set up one at a time and for any one such setting up process the combination of cold cathode'thyratrons to be operated is indicated to all the slaves from a master selector which also includes two groups one of 9 and one of 11 cold cathode thyratrons. When a connection is set up a combination of cold cathode thyratrons in the master selector is operated in a manner to be described later and the resulting rise in the potential of the cathodes is applied to the strikers of the corresponding cold cathode thyratrons in all the slaves. The slave which is to be operated by the indicated combination of signals from .the master selector is indicated by the presence of a 10 kc./ sec. hold signal on either of the two signalling pairs, one for each direction of transmission as already described with reference to Fig. 11 and appearing in the slave selector shown in Fig. 12 over pair circuits 13a and 16a. The presence of a ll) kc./sec. hold signal'on either of these pair circuits is used to lower thepotehtial of the cathodes of all the cold cathode thyratrons in the slave selector so that the combination of thyratrons which is marked by the master selector will have sufficient voltage between striker and cathode to initiate a discharge between the anode and cathode. The slave selector will then generate the required selecting pulses and in a manner to be described later this will cause the release of the master selector which may then be used to set up another connection through the switch using some other free slave selector. Once one cold cathode thyratron in each group has been operated the potentials of the cathodes of the remaining cold cathode'thyratrons in the slave selector rise sufficiently to prevent any further cold cathode thyratrons in the slave from being operated bysubsequent marking from'the m'asterselector.

The potential of the strikers of the slaves is normally at 30 v. above earth potential and this rises to 80v.

when the associated cold cathode thyratron in the master selector is operated. To limit transfer current these potentials are applied to the strikers of the slave cold cathode thyratrons through resistors: Threeleads froiii the master selector are shown in Fig. 12. Lead 22 which 18 is'tassociated with pulses 1W; is applied to the striker of =V19 throughzr'esistor R27, lead 23 which is associated withipulsesPg is applied to the striker of V20 through resistor R28 and lead 24 which is associated with pulses P i 'is applied to striker of V22 through resistor R30.

The cathodes Within each group of cold cathode thyratrons in a slave are connected together. Thus lead 20 is connected to all the cathodes in the group of 9 thyratrons and lead 21 is connected to all the cathodes in the group of 11 thyratrons. When no hold signal is applied over either signalling pair circuits 16a or 13a the potential of the cathode leads 20 and 21 is approximately 50 volts above earth potential. This potential is derived for lead .20 through resistor R and the forward resistances of the rectifiers W and W48 in parallel with the forward resistances of rectifiers W52 and W54 which are all biased to conduct by the +100 volt supply applied throughthe high resistance resistor R47. Similarly lead 21 derives its potential through resistor R46 and therectifiers W52, W53, W54- and W55. The voltage between striker and cathode of the thyratrons is now inadequate to initiate a current discharge in any of the cold cathode thyratrons even though the potential of some of the strikers may be volts above earth potential. The voltage between the cathode and anode is less than the stabilising voltage of the thyratrons so that no cold cathode thyratrons are conducting and the slave is in its non-operated state.

The slave which is to become operated is prepared by the presence of a hold signal on either of pair circuits 13a or 1611. This is a 10 kc./sec. signal and is applied to the primary winding of the transformers T18 and T19. The operation for a hold signal applied over 16a, which as will be described later is the signal used in so called line-finder action, will now be described. The operation for a hold signal applied over 13a, as used in so-called selector action, is the same except that it is initiated by A. C. power over circuit 13:: in place of power over circuit 160. The transformer T18 has two secondary windings, sheet which is floating and substantially balanced and isconnected via capacitors C37 and C38 to voltage doubling rectifier circuits. Thus one end of the balanced winding is applied via capacitor C37 to a voltage doubling circuit well known to those skilled in the art andconsisting of the rectifiers W48 and W50 and the capacitor C41. The other end of the winding is applied via C38 to a voltage doubling circuit consisting of the rectifie'rs W49 and W51 and the capacitor C42. The capacitors C41 and C42 are common to the voltage doublers associated with both the hold signals. The applied voltage charges C37 and C33 in series so that the resulting D. C. potential which is applied to the cathodes is approximately equal to the peak to peak voltage appearing across the transformer secondary winding. .This is made substantially-equal to 50 v. so that the cathode potentials fall to approximately earth potential. The voltage between the cathodes and the strikers of the cold cathode thyratrons forming the combination marked from the master selector is then approximately 80 volts and this is sufficient to initiate an anode-cathode discharge and therefore to operate the slave to generate the required selector pulse.

The current taken by the operated cold cathode thyratrons causes a rise in the potential of the cathodes across resistors R45 and. R46 and the output impedance of the voltage doubling circuit to a positive potential approximately 20 volts above earth potential. This voltage is sufiicient'to prevent the striking of any other cold cathode thyratrons in either group but will clearly not rise so much that the voltage between anode and cathode is less than the stabilising voltage of the cold cathode thyratron.

A feature of the circuit is that if for any reason a cold cathode thyratron in one group operates before one in the other group due to variations the times of application of thesignal from the master selector or due to unbalance of the transformer windings which will cause unequal changes in the cathode potentials of the two groups, the load which the operated cold cathode thyratron will put on one side of the secondary will automati cally unbalance the transformer and by limiting the voltage swing on one side of the floating winding, will increase the voltage swing on the other so that the cathodes of the unstruck group will tend to be further depressed thus assisting the operation of the unoperated but marked cold cathode thyratron.

To prevent the cathode potentials from going too far negative with respect to earth, which may cause the false operation of a cold cathode thyratron, two gas discharge tubes V25 and V26 connected one to each of the common cathode leads 20 and 21 and to the +50 volt supply prevent the potentials of either group of cathodes becoming more than 15 volts below earth potential. The normal striker potential of 30 volts will thus not cause the false operation of a cold cathode thyratron.

When a cold cathode thyratron has been operated the current taken will introduce a voltage ripple on the cathode lead. This cannot be eliminated by increasing the size of capacitors C41 and C42 without increasing the delay in building up the required change in cathode potential. The ripple is reduced by inserting capacitors C43 and C44 between the cathodes and earth potential and the effect on the anode gate circuits which may introduce variations in the amplitudes of the slave selector pulses is made negligible by connecting the anodes of the cold thyratrons to earth via capacitors. This is shown in Fig. 12 in which capacitors C29, C30 C34 are connected to the anodes of cold cathode thyratrons V19, V20 V24 respectively. These capacitors also minimise the effect of current fluctuations in the operated cold cathode thyratrons.

The presence of the holding signal on either of pair circuits 1611 or 13a will maintain the slave in its operated state and until both signals have disappeared the slave continues to generate the required selecting pulses. Upon release of the connection through the switch both these hold signals disappear and the cathode potentials of the cold cathode thyratrons will rise until the thyratrons are extinguished. The cathode potentials continue to rise as the capacitors C41 and C42 are charged through R47 and R48 until the voltage doubling rectifiers are again conducting and the cathode potentials are again 50 volts above earth potential.

The presence of a hold signal may be used to indicate that the slave selector and its associated circuit are busy. The second secondary windings on transformers T18 and T19 are provided for this purpose and the output of these windings may be rectified by well known means using the rectifiers W56 and W57 and the capacitor C45 common to both hold signals. The lead 25 shown in Fig. 12 is the slave busy indicating lead and when the slave is free its potential is 10 volts above earth potential. The rectified l kc./sec. hold signal causes this potential to fall to earth and as will be described subsequently this signal may be used to prevent the re-selection of a busy slave selector. The capacitor C45 is made sufliciently large to continue the busy condition after the hold signals have been removed until the cold cathode thyratrons are completely extinguished. The resistors R49 and R50 in series with the rectifiers W56 and W57 are inserted to limit the load which capacitor C45 would present to the hold signal source.

It has already been described how the cold cathode thyratrons in the slave selectors are struck by the application of a D. C. signal to the striker of the selected cold cathode thyratrons and the striking of two cold cathode thyratrons in a slave selector causes a selecting pulse train to be produced.

D. Master selector (Fig. 14)

It will now be described with reference to Fig. 14, how

the selecti'on'of the two cold cathode thyratrons in the slave selector which are struck to produce a selecting pulse which coincides withparticular multiplex pulses, is accomplished. The master selector includes two groups of'registers which in the embodiment of Fig. 12 comprise cold-cathode thyratrons. There are 9 tubes in one group and 11 in the other and the twenty cold cathode thyratrons are each associated with a particular 9 or 11 phase pulse train and each has a coincidence gate circuit connected to its striker through a pulse lengthener. Twenty leads, one from the cathode of each master selector cold cathode thyratron are connected to the strikers of corresponding cold cathode thyratrons in the slave selectors.

One such master selector cold cathode thyratron coincidence gate circuit and pulse lengthener comprises valves V27, V28 and V29 and their associated circuit. The operation of this will be described. Capacitors C48 and C49, rectifier W60 and resistors R55, R54 form a coincidence gate circuit the output of which is applied to the control grid of valve V29. Capacitor C48 is connected to pulses which are negative going 9-phase pulses of 0.8 microand the set up pulse are derived from a low impedance source so that capacitors C48 and C49 will be charged in series through the forward resistance of rectifier W60 by the negative going pulses P-gn Capacitor C49 has approximately ten times the cappacitancc of capacitor C48 and as capacitor C49 must discharge through the reverse resistance of rectifier W60, after a few pulses it becomes charged to the peak amplitude of pulses this voltage decaying only slightly between successive pulses. When this condition is reached charging takes place only on the peaks of the pulses when sufiicient current flows to replace the charge lost by capacitor C49 between the pulses, and at other times the rectifier W60 will be biased to its non conducting condition by the voltage to which capacitor C49 is charged which is the peak amplitude of the pulses. Capacitor C48 discharges through resistor R55 between pulses and resistor R55 is of such a resistance that capacitor C48 does not build up a permanent charge. Thusthe potential at the junction of resistors R54 and R55 is normally that of the 6 v. supply to which resistor R55 is connected, dropping by the amplitude of the pulses on the incidence of these pulses and returning to 6 volts below earth potential between the pulses. The 

